The Gameboy CPU looks more like a stripped-down Z80 to me, it doesn't have the ED, DD and FD extended instruction ranges of the Z80 (most of those aren't that useful anyway), and the shadow register set is also missing, but the complete CB instruction range exists (bit twiddling and additional shift/rotate instructions). Also the relative jump instructions of the Z80 are there which are redundant NOPs in the 8080 instruction set.
I think the missing IX and IY are definitive Z80 instructions. The 'Ultimate Gameboy talk' has a nice Venn diagram and they tend to agree that the GB is missing the 'interesting' instructions from the Z80. However the added shifts etc from Z80 are quite useful. But on the continuum between 8080 to Z80 the GB80 is closer to 8080
